An undercarriage of a truck vehicle typically comprises a suspension system that includes suspension of an axle, such as a rear axle, from an undercarriage component, such as a chassis frame. The suspension system absorbs road-induced impacts as the vehicle travels, contributing to quality of vehicle ride and vehicle handling.
One type of suspension system is a steel spring suspension system. Such a system typically comprises a multi-leaf spring on the right side of the vehicle and a multi-leaf spring on the left side of the vehicle. Front and rear ends of the longest leaf in each spring are mounted on a respective undercarriage component, such as a chassis frame side rail, via respective front and rear spring leaf mounting brackets fastened to the respective undercarriage component. A shackle may be present between one end of the longest leaf and the corresponding mounting bracket. An axle is fastened to the suspension system by fastening a housing of the axle to each multi-leaf spring midway between the respective front and rear mounting brackets.
As the vehicle travels, irregularities in an underlying road surface impart vertical forces to the axle. While those forces are cushioned in part by the suspension system, they are reacted on the undercarriage via the spring leaf mounting brackets.
The nature of the mounting of a steel spring suspension system to an undercarriage can inherently maintain proper alignment of the axle to the undercarriage and to wheels other than those on the axle. Proper alignment of a drive axle to the undercarriage provides straight-line travel that avoids creating a non-zero, or off-center, thrust angle that creates a condition sometimes referred to as “dog tracking”, a condition that contributes to steering wheel misalignment because the driver must over- or under-steer to correct for it. Dog tracking can lead to accelerated tire wear.
Another type of suspension system is an air suspension system. Instead of steel spring leaves, an air suspension system has air springs on right and left sides of the axle for absorbing vertical road forces imposed on the axle. An air spring typically has a single point of attachment to an undercarriage, unlike the front-rear mounting of a spring leaf via front and rear spring leaf mounting brackets. Maintenance of proper alignment of the axle to the undercarriage is provided by elements of the suspension system other than the air springs because the air springs cannot do so by themselves.